Selection of nonequilibrium overlimiting currents: universal depletion layer formation dynamics and vortex instability.

The ion flux dynamics across a straight nanoslot is imaged to understand the nonequilibrium phenomenon of overlimiting current density across a nanoporous membrane. With a slow ac field, an ion-depletion front is generated intermittently from one end of the nanochannel, and a vortex instability first predicted by Rubinstein, Staude, and Kedem [Desalination 69, 101 (1988).10.1016/0011-9164(88)80013-4] is found to arrest the self-similar diffusive front growth. This electrokinetic instability evolves into a stationary interfacial vortex array that specifies the overlimiting current, independent of external stirring or convective flow.